1. Field of the Invention
The present invention relates to a novel substance that efficiently absorbs lights of near infrared wavelength range.
2. Description of the Related Art
Materials that absorb lights of near infrared wavelength range are utilized as heat ray absorbers or optical filters.
Various materials including those mentioned below have been examined so far.
For example, metal iron oxides such as ferrous oxide are known to absorb heat rays. However, since their absorption coefficient is considerably low, and they suffer from a problem of poor solubility in solvents. Therefore, they are not practically useful. Further, copper carboxylates as organic copper salts such as copper benzoate, copper acetate and copper naphthenate are also known to absorb heat rays. However, they have a maximum absorption wavelength of around 600-750 nm, and they only show weak absorption in near infrared range.
It has also been reported that a water-based solution added with ferrous sulfate or ferrous ammonium sulfate absorbs lights of near infrared wavelength range (Japanese Patent Laid-open (Kokai) Nos. 63-116625/1988, Japanese Patent Laid-open (Kokai) Nos. 63-116626/1988). However, if a solution of these sulfates is left for a while, they react with water to generate deposition or precipitation. Therefore, they suffer from a problem of troublesome handling, i.e., a suitable amount of sulfuric acid must be added to the solution in order to avoid such deposition or precipitation.
It is noted that phthalocyanine and naphthalocyanine are known as copper-containing organic compounds absorbing near infrared-rays. However, both of these show absorption in visible light range. Further, they have a maximum absorption wavelength of 800 nm or shorter with a sharp peak, and hence they do not show absorption in broad wavelength range.
Furthermore, there has been proposed an optical filter absorbing lights of near infrared range, characterized in that it comprises a copolymer obtained by copolymerizing a monomer with a specific structure having a phosphoric acid group and a monomer copolymerizable with the former monomer, and a metal salt mainly consisting of a copper salt (Japanese Patent Laid-open (Kokai) No. 6-118228/1994). There have also been proposed a near infrared absorbing resin composition comprising a resin obtained by polymerizing a monomer having an unsaturated double bond, a phosphorus atom-containing compound with a specific structure and copper hydroxide, and a near infrared absorbing resin composition comprising a phosphorus atom-containing compound with a specific structure (Japanese Patent Laid-open (Kokai) Nos. 10-152598/1998 and 10-153964/1998). Although these compositions absorb lights of near infrared wavelength range, they suffer from a problem of low water solubility.
An object of the present invention is to provide a compound absorbing near infrared rays, which does not substantially absorb lights of visible light wavelength range, but strongly absorb lights of near infrared wavelength range, and shows extremely good water solubility.
The inventors of the present invention assiduously studied in order to achieve the aforementioned object. As a result, they found that a copper salt of naturally occurring phytic acid strongly absorbs lights of near infrared wavelength range, and thus accomplished the present invention.
That is, the present invention provides the followings.
(1) A copper salt of phytic acid, which is formed from copper ion (Cu2+) and phytic acid at a molar ratio of copper ion to phytic acid of 2:1 or higher.
(2) The copper salt of phytic acid according to (1), which is obtainable by reacting an inorganic copper salt or an organic copper salt with phytic acid.
(3) The copper salt of phytic acid according to (2), wherein the organic copper salt is a copper salt of a C2-C8 carboxylic acid.
(4) A near infrared absorber comprising a copper salt of phytic acid according to any one of (1) to (3).